1. Field of the Invention
The present invention relates to an optical pickup device, and an optical recording and reproducing apparatus including this optical pickup device (this optical recording and reproducing apparatus contains a magneto-optical recording and reproducing apparatus) and a gap detection method, and more particularly to an optical pickup device, an optical recording and reproducing apparatus and a gap detection method for use with a so-called near-field optical recording and reproducing system for recording and/or reproducing an optical recording medium while an optical lens is increasing its numerical aperture.
2. Description of the Related Art
Optical recording mediums, which are typically available in the form of a compact disc (CD), a mini disc (MD) and a digital video disc (DVD), have been so far widely used as storage mediums for storing therein music information, video information, data, programs and the like (“optical recording medium” in this specification refers to not only the optical recording medium but also a “magneto-optical recording medium”).
These optical recording mediums can be recorded and/or reproduced by laser beams irradiated on the signal recording surface of the optical recording medium from the optical pickup device. Specifically, when the optical recording medium is reproduced by an optical pickup device according to the related art, for example, very small changes of reflectance are read out from pits formed on one side of the optical recording medium through a non-contact objective lens, which is not in contact with the optical recording medium, such as an objective lens of a microscope. In the magneto-optical detection, miniscule magnetic domains are read out from the magneto-optical disk based upon a Kerr rotation angle.
The diameter of beam spot on the optical disk is roughly given by λ/NA (where λ represents the wavelength of illumination light and NA represents the numerical aperture of the lens), and resolution is proportional to the value given by this equation. With respect to the numerical aperture NA, the following equation is established:NA=n·sin θ(where n represents the refractive index of the medium and θ represents the angle of rays of light around the objective lens).
When the medium is air, the numerical aperture NA is inhibited from exceeding 1. As a technology that can surpass this limit, there has been proposed an optical pickup device of a near-field optical recording and reproducing system using “solid immersion lens (see the following non-patent document 1).
The solid immersion lens (SIL) has the same refractive index as that of an optical disk substrate, and is shaped like a part of a sphere including a spherical surface portion and a flat surface portion, the flat surface portion thereof being very close to the surface of the optical recording medium. Evanescent wave transmits through the boundary surface between the solid immersion lens and the optical disk, and this evanescent wave reaches the signal recording surface of the optical disk.
Non-Patent Document 1:
I. Ichimura et. al, “Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture.” Jpn. J. Appl. Phys. Vol. 39, 962-967 (2000)
Since it is customary for the above-mentioned optical pickup device to read an information signal from the optical disk while the optical disk is being rotated, the optical disc and the solid immersion lens have to require a gap (space) therebetween. Therefore, the evanescent wave should be used in order to achieve the numerical aperture NA greater than 1. Since the evanescent wave attenuates exponentially from the interface, the gap between the optical disk and the solid immersion lens should be made extremely thin, i.e., approximately 10/1 of the wavelength λ of light from the light source of the optical pickup device, for example. Moreover, in order to reduce the area of the gap, the gap has to approach the signal recording surface of the optical disk.
As a method for controlling the above gap, there has hitherto been proposed a method for servo-controlling a distance (gap) between a solid immersion lens and an optical disk based upon a gap error signal after the gap error signal has been obtained by detecting an electrostatic capacity between an electrode formed on the surface of the solid immersion lens and the optical disk.
However, in order to execute this previously-proposed method, the electrode has to be formed on the surface of the solid immersion lens and a signal line has to be led from this electrode to a control circuit so that the optical pickup device becomes complex in arrangement, and hence it becomes difficult to manufacture the optical pickup device.
On the other hand, in the mastering process of the optical disk, the assignee of the present application has previously proposed a method for using returned light detected from a glass master disk as a gap error signal (see Japanese patent application No. 10-249880).
When the solid immersion lens and the glass master disk have no gap therebetween, the surface of the solid immersion lens is in contact with the transparent photoresist on the glass master disk, and hence no light is returned from the surface of the solid immersion lens. Conversely, when the solid immersion lens and the glass master disk have the gap therebetween, light that has been totally reflected on the surface of the solid immersion lens is returned. The above previously-proposed method is able to detect the gap between the solid immersion lens and the glass master disk by using this returned light.
However, this previously-proposed method can be used only when the master disk is made of glass and the photoresist for use in exposure is transparent. If the master disk has a reflective film such as an aluminum film, a phase-change film and a magneto-optical recording film deposited on its surface like an optical disk, then even when the solid immersion lens and the master disk have no gap therebetween, light is reflected on the surface of the optical disk and no light is returned. Hence, this previously-proposed method cannot be used.
In order to solve the above-mentioned problem, the assignee of the present application has previously proposed an optical pickup device that can accurately detect a very small gap between an optical disk with a reflective film deposited on its surface and a solid immersion lens (see Japanese patent application No. 2001-264467).
The above previously-proposed optical pickup device will be described below with reference to the drawings.
FIG. 1 of the accompanying drawings is a side view showing an arrangement of an example of this optical pickup device.
As shown in FIG. 1, the optical pickup device includes an objective lens 2 composed of a solid immersion lens (SIL) 1 having a spherical portion and a flat surface portion parallel to the surface of an optical recording medium 90 to form a part of a shape of a sphere, the objective lens 2 having a numerical aperture greater than 1. The solid immersion lens 1 is shaped like a hemisphere, for example, and has a thickness substantially equal to the radius of the sphere. A distance (gap) between the flat surface portion of the solid immersion lens 1 and the surface of the optical recording medium 90 can be held at approximately 10/1 of the wavelength of light emitted from a semiconductor laser 3 serving as a light source under control of a servo mechanism which will be described later on.
This optical pickup device is able to obtain a gap error signal corresponding to a distance between the surface of the optical recording medium 90 and the flat surface portion of the solid immersion lens 1 by detecting a component of the polarized state perpendicular to the polarized state of reflected light obtained when the surface of the optical recording medium 90 and the flat surface portion of the solid immersion lens 1 have no gap therebetween from reflected lights (returned lights) reflected on the optical recording medium 90 after they have been emitted from the semiconductor laser 3.
Specifically, in this optical pickup device, a bundle of rays emitted from the semiconductor laser 3 is collimated by a collimator lens 4 as a bundle of parallel rays and introduced into a beam splitter 5. A bundle of rays emitted from the semiconductor laser 3 has a wavelength of 400 nm, for example. A bundle of rays emitted from the semiconductor laser 3 transmits through the beam splitter 5 and becomes incident on a polarization beam splitter 6. A bundle of rays emitted from the semiconductor laser 3 is a p-polarized light relative to the reflection surface of the polarization beam splitter 6, and hence it transmits through the reflection surface of the polarization beam splitter 6, whereafter it transmits through the polarization beam splitter 6.
A bundle of rays that has transmitted through the polarization beam splitter 6 transmits through a λ/4 plate (quarter-wave plate) 7 with its crystallographic axis inclined at an inclination angle of 45° relative to the direction of the incident polarized light, by which it is double-refracted so as to become circularly polarized light and the circularly polarized light is introduced into an objective lens 8 comprising the condenser lens 2 together with the solid immersion lens 1. A bundle of incident parallel rays is converged by this objective lens 8 and introduced into the solid immersion lens 1. This solid immersion lens 1 has a focal point formed near a parallel portion disposed parallelly close to the surface of the optical recording medium 90. A refractive index of the solid immersion lens 1 is selected to be 1.8, for example.
A bundle of the thus converged rays is converged on the signal recording surface of the optical recording medium 90 as evanescent wave. In this case, the objective lens 2 has an NA (numerical aperture) of approximately 1.36, for example.
The optical pickup device shown in FIG. 1 is constructed as the optical pickup device capable of reproducing either an optical disk on which an information signal is recorded by recording pits (concavities and convexities) or a recordable optical disk on which an information signal is recorded by using phase-change. Specifically, a bundle of rays that has been converged on the signal recording surface of the optical recording medium 90 is reflected in various manners with or without application of the recording pits on this signal recording surface and returned to the polarization beam splitter 6 through the objective lens 2 and the quarter-wave plate 7.
A bundle of rays returned to the side of the condenser lens 3 after it has been reflected on the surface of the optical recording medium 90 is introduced into the quarter-wave plate 7, in which it is double-refracted in the form of circularly polarized light to linearly polarized light. At that time, the direction of polarized light is normal to the direction of polarization of a bundle of rays emitted from the semiconductor laser 3. Accordingly, a bundle of rays returned after it has been reflected on the surface of the optical recording medium 90 is s-polarized light relative to the reflection surface of the polarization beam splitter 6 and thereby reflected on the reflection surface of the polarization beam splitter 6 so that it is deviated from the light path along which it may return to the semiconductor laser 3, thereby being received at a first photo-detector 9 which is used to obtain a reproduced signal from the optical recording medium 90.
According to this optical pickup device, on the surface A between the beam splitter 5 and the polarization beam splitter 6, a bundle of rays emitted from the semiconductor laser 3 is linearly polarized light containing only an electric field component of X direction as shown in FIG. 2A but which does not contain an electric field component of Y direction as shown in FIG. 2B.
Then, in the state in which the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90, this flat surface portion is in close contact with a phase-change recording and reproducing multilayer film (composed of Al film, SiO2 film, GeSbTe film, SiO2 film deposited on the substrate, in that order) deposited on the surface of the optical recording medium 90 as shown in FIG. 3A or a reflective film made of a suitable material such as aluminum (Al film deposited on a substrate) deposited on the surface of the optical recording medium 90 as shown in FIG. 3B.
As described above, in the state in which the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90, most of reflected light is inwardly and outwardly transmitted through the quarter-wave plate 7 and thereby the reflected light is double-refracted so as to have the direction of polarization rotated 90° so that a bundle of rays with a distribution substantially equal to that of light emitted from the semiconductor laser 3 may become incident on the surface B that is the surface just ahead of the first photo-detector 9 as shown in FIG. 4A. At that time, as shown in FIG. 4B, reflected light is hardly returned from the optical recording medium 90 to the surface A between the beam splitter 5 and the polarization beam splitter 6.
Then, in the state in which the solid immersion lens 1 is away from the optical recording medium 90, as shown in FIG. 5, of light beams converged near the flat surface portion of the solid immersion lens 1, a light beam that will become incident on the flat surface portion at an incidence angle greater than a critical angle in this flat surface portion is reflected on the flat surface portion ((refractive index of solid immersion lens)×sin (incidence angle)>1).
In the thus reflected light, its direction of polarization is rotated delicately when it is totally reflected. The light beam that has been totally reflected as described above contains a polarized light component perpendicular to the reflected light obtained in the state in which the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90 as described above. As a result, a distribution of the returned light on the surface A between the beam splitter 5 and the polarization beam splitter 6 becomes a distribution in which only light beams at the portions corresponding to the marginal portions of a bundle of rays are returned as shown in FIG. 6A.
The light beam that has returned to the surface A as described above is reflected on the reflection surface of the beam splitter 5 and received at a second photo-detector 10 which is used to obtain a gap error signal as shown in FIG. 1. This gap error signal is a signal corresponding to a distance between the flat surface portion of the solid immersion lens 1 and the optical recording medium 90.
Then, at that time, a distribution of the returned light beam on the surface B located just ahead of the first photo-detector 9 becomes a distribution in which returned light beams at the portions corresponding to the marginal portions of a bundle of rays are missed.
In a relationship between a quantity of light received at the second photo-detector 10 and the distance (air gap) between the flat surface portion of the solid immersion lens 1 and the surface of the optical recording medium 90, this distance (air gap) can be held at 10/1 of the wavelength by controlling the position at which the solid immersion lens 1 comes in contact with or comes away from the optical recording medium 90 in such a manner that the quantity of light at the second photo-detector 10, for example, may be kept at the ratio of quantity of incident light of 0.2 as shown in FIG. 7.
When the optical recording medium 90 is a magneto-optical disk, an optical pickup device having an arrangement shown in FIG. 8, for example, can be applied to this example. Specifically, as shown in FIG. 8, a bundle of rays emitted from the semiconductor laser 3 is converged on the signal recording surface of the optical recording medium 90 through the collimator lens 4, the polarization beam splitter 6, the beam splitter 5, the condenser lens 8 and the solid immersion lens 1. In this optical pickup device, a quarter-wave plate is not provided on the outward light path to the optical recording medium 90.
Then, the returned light beam that has been reflected on the optical recording medium 90 is divided by the beam splitter 5, whereafter it is refracted by a λ/2 plate (half-wave plate) 11 (i.e., polarization filter) so that its direction of polarization is rotated 45° and introduced into a second polarization beam splitter 12. This half-wave plate 11 is disposed with its optical axis inclined 22.5° relative to the direction of incident linearly polarized light.
The light beam that became incident on the second polarization beam splitter 12 is divided in response to a Kerr rotation angle generated based on a magneto-optical effect when it is reflected on the signal recording surface of the optical recording medium 90, and it is received at the first and second photo-detectors 13 and 14, both of which are used to obtain a magneto-optical signal. A difference signal between outputted signals from the first and second photo-detectors 13, 14 is not generated when a bundle of reflected rays does not generate the Kerr rotation angle and becomes an output corresponding to the Kerr rotation angle generated in a bundle of reflected rays, which becomes a magneto-optical signal.
Then, a bundle of rays that has returned from the flat surface portion of the solid immersion lens 1 in order to obtain the gap error signal is returned through the beam splitter 5 to the polarization beam splitter 6. Then, it is reflected by this polarization beam splitter 6 and received at the third photo-detector 10 that is used to obtain the gap error signal.
When the optical recording medium 90 is the magneto-optical disk, the optical pickup device may be modified as an optical pickup device having an arrangement shown in FIG. 9, for example. Specifically, as shown in FIG. 9, a bundle of rays emitted from the semiconductor laser 3 may be converged on the signal recording surface of the optical recording medium 90 through the collimator lens 4, the beam splitter 5, the condenser lens 8 and the solid immersion lens 1.
The light beam that has been irradiated on the signal recording surface of the optical recording medium 90 in this manner is reflected on this signal recording surface and reflected on the beam splitter 5, whereafter it is further divided by the second beam splitter 15 into two bundles of rays. A bundle of rays that has passed the second beam splitter 15 is transmitted through the half-wave plate 11 and thereby its direction of polarization is rotated 45°, whereafter it is introduced into the polarization beam splitter 12. This half-wave plate 11 is located such that its optical axis is inclined at an inclination angle of 22.50 relative to the direction of incident linearly polarized light.
The light beam that has been introduced into the polarization beam splitter 12 is divided in response to the Kerr rotation angle generated by the magneto-optical effect when it is reflected on the signal recording surface of the optical recording medium 90 and received at the first and second photo-detectors 13 and 14 which are used to obtain the magneto-optical signal. A difference signal between the outputted signals from the first and second photo-detectors 13, 14 is not generated when a bundle of reflected rays does not have the Kerr rotation angle, and becomes an output corresponding to the Kerr rotation angle generated in this bundle of rays, which becomes a magneto-optical signal.
On the other hand, a bundle of rays reflected by the second beam splitter 15 is introduced into the second polarization beam splitter 16. Of this bundle of rays, a bundle of rays that has returned from the flat surface portion of the solid immersion lens 1 in order to obtain the gap error signal is reflected by the second polarization beam splitter 16 and received at the third photo-detector 10 that is used to obtain the gap error signal.
Also in the optical pickup device shown in FIG. 9, similarly to the case of the optical system shown in FIG. 1, the polarized state of incident light introduced into the beam splitter 5 becomes linearly polarized light that contains only the electric field component of the X direction as shown in FIG. 2A but that does not contain the electric field component of the Y direction as shown in FIG. 2B. Each of the beam splitters 5, 15 transmits and reflects the polarized components of both X, Y directions equally.
When the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90, this flat surface portion is in close contact with the phase-change recording and reproducing multilayer film (composed of Al film, SiO2 film, TeFeCo film, SiO2 film deposited on the substrate, in that order) deposited on the surface of the optical recording medium 90 as shown in FIG. 10. A distribution of returned light on the surface B which is the surface required after the returned light has passed through the second polarization beam splitter 16 at that time becomes a distribution substantially equal to those of light beams emitted from the semiconductor laser 3 as shown in FIG. 11A. Then, in the surface C which is the surface disposed just before returned light is reflected by the second polarization beam splitter 16 and introduced into the third photo-detector 10, the returned light from the optical recording medium 90 is hardly returned as shown in FIG. 11B. Accordingly, when the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90, returned light on the surface C is almost zero, and hence returned light hardly reaches the third photo-detector 10.
Then, in the state in which the solid immersion lens 1 is distant from the optical recording medium 90, as shown in FIG. 5, of light beams converged near the flat surface portion of the solid immersion lens 1, light incident on the flat surface portion of the solid immersion lens 1 at an angle exceeding the critical angle of this flat surface portion is totally reflected on this flat surface portion of the solid immersion lens 1 ((refractive index of solid immersion lens)×sin(incidence angle)>1)
The thus totally reflected light delicately rotates its direction of polarization when it is totally reflected. Then, the thus totally reflected light contains a polarized component normal to the reflected light obtained in the state in which the flat surface portion of the solid immersion lens 1 is in close contact with the surface of the optical recording medium 90 as described above. As a result, a distribution of the returned light on the surface C which is the surface required just before the light is reflected by the second polarization beam splitter 16 and introduced into the third photo-detector 10 becomes a distribution in which the portions corresponding to the marginal portions of a bundle of rays are partly returned as shown in FIG. 12B.
The light thus returned to the surface C is received at the second photo-detector 10 which is used to obtain a gap error signal. This gap error signal is a signal corresponding to the distance between the flat surface portion of the solid immersion lens 1 and the surface of the optical recording medium 90.
Then, at that time, a distribution of the returned light on the surface B which is the surface behind the second polarization beam splitter 16 becomes a distribution in which the portions corresponding to the marginal portions of a bundle of rays are missing as shown in FIG. 12A.
In a relationship between the quantity of light received at the third photo-detector 10 and the distance (air gap) between the flat surface portion of the solid immersion lens 1 and the surface of the optical recording medium 90, as shown in FIG. 13, this distance (air gap) can be held at 10/1 of the wavelength of the light by controlling the position of the direction in which the solid immersion lens 1 comes in contact with or comes away from the optical recording medium 90 such that the quantity of light of the third photo-detector 10, for example, is held at the ratio of quantity of incident light of 0.1.
However, when these previously-proposed optical pickup devices are in use, the arrangement shown in FIG. 1 requires a plurality of beam splitters such as the beam splitter 5 for obtaining the gap error signal and the polarization beam splitter 6 for obtaining the reproduced signal and also requires separately independent photo-detectors such as the photo-detector 10 for obtaining the gap error signal and the photo-detector 9 for obtaining the reproduced signal. There arises a problem, in which the optical pickup device becomes complex in arrangement. The optical pickup devices shown in FIGS. 8 and 9 needs much more beam splitters and photo-detectors. Further, a problem arises, in which the recording and reproducing apparatus requires the optical pickup device having the complex arrangement so that the recording and reproducing apparatus which assembles such optical pickup device also becomes complex in arrangement.